Consumer Units
What are consumer units?
Consumer units have been called m any different names over the years, such as fuse box, fuse board
and dis-board. They are typically installed in domestic premises to provide control, distribution and
protection to the various final circuits within the installation.
What are the requirements of BS 7671:2018 today, for consumer units in dwellings?
Over the years the requirements have changed drastically, in the 1960s it was acceptable to install a
consumer unit with a wooden back box, fuses were typically, the rewireable type providing protection
against overcurrent and short-circuit and protection from electric shock, but what is required in the
current edition of the IET Wiring Regulations, BS 7671:2018?
Protection from spread of fire
From January 2016, The IET Wiring Regulations required that consumer units in domestic
(household) premises were manufactured from non-combustible material. The intent is to contain any
fire within the enclosure, to minimise the spread of fire and to remove an additional source of fuel for
the fire, i.e. the plastic material. There is substantial evidence to show that if there is a deficiency
within a plastic consumer unit, such as, a loose connection, the y can burn and release toxic gases.
This could have more severe consequences if the consumer unit is installed on an escape route, for
example, in a hallway or under the stairs.
Circuit protection
Within the enclosure itself, circuit protective devices are installed to protect the final circuits. T ypically,
there is a main switch which serves as an isolation device only and does not offer any electrical
protection. However, this could be used in an emergency to isolate the whole installation, for example,
if someone was receiving an electric shock or overheating was evident. Fuses provide protection
against overcurrent and short-circuit faults and the modern equivalent is the circuit-breaker. These
devices are rated in amps (A) and there will be one installed per final circuit within the installation, for
example, cooker, shower, socket-outlets and lights.
Protection from electric shock
Generally, all circuits within the domestic installation will have requirements for additional protection
by means of a Residual Current Device (RCD). RCDs are required to protect people from electric
shock and the rating of these devices is stated in milliam ps (mA), so they are thousands of times
more sensitive than circuit-breakers or fuses. The usual arrangement is for two RCDs to each protect
a number of circuits within the installation. This is to prevent danger or inconvenience to the user in
the event of a fault. For example, where upstairs and downstairs lighting circuits are split over two
separate RCDs, if one RCD operates, the other circuit will continue to operate thus preventing the
user from being plunged into complete darkness. RCDs incorporate a test button to prove the
mechanical operation of the device and should be tested at a maximum frequency of six months.
Protection from fire
RCDs will also provide some protection from fires arising from insulation faults within the installation.
There is a recommendation in the latest edition of the IET Wiring Regulations, BS 7671:2018, to
incorporate Arc Fault Detection Devices (AFDD) to protect from fire. Please see separate article on
AFDD.
Protection from surge
A new regulation in the latest edition of the IET Wiring Regulations has a requirement to incorporate
protection against surges. Please see the separate article on SPDs here.
